1. Technical Field
The present invention relates to an image forming apparatus and an image forming method in which transfer is performed in a state of gripping a transfer medium.
2. Related Art
In the past, among image forming apparatuses, there has been proposed an image forming apparatus in which a transfer roller having a transfer medium gripping member that grips an edge portion of a transfer medium is used (see, for example, JP-T-2000-508280). In the image forming apparatus disclosed in JP-T-2000-508280, by the rotation of the transfer roller, the transfer medium passes through a transfer nip in a state where the apical portion of the transfer medium is gripped by the transfer medium gripping member, and an image on an image carrier is transferred to the transfer medium. After the apical portion of the transfer medium passes through the transfer nip, the gripping of the transfer medium is released and thus the transfer medium is released. According to this image forming apparatus, the transfer medium is reliably detached from the image carrier after the transfer by gripping the apical portion of the transfer medium by the transfer medium gripping member.
On the other hand, there is proposed an image forming apparatus in which, by the rotation of the transfer roller, the transfer medium passes through the transfer nip and the image of the image carrier is transferred to the transfer medium, and when the apical portion of the transfer medium passes through the transfer nip the transfer medium is detached from the image carrier by an airflow generating device, and then the transfer medium is moved from a downward position to an obliquely upward position in a state where the transfer image plane thereof is directed downward, and is transported to a fixing section side by a transfer medium transporting belt of a transfer medium transporting section (see, for example, JP-A-2009-205131).
Incidentally, it is considered that a technique for gripping the apical portion of the transfer medium disclosed in JP-T-2000-508280 is applied to the image forming apparatus disclosed in JP-A-2009-205131, in order to improve detachability of the transfer medium from the image carrier after the transfer. In this case, since the gripping of the apical portion of the transfer medium is released just before the transfer medium transporting section, a nip termination position of the transfer nip in a transfer medium moving direction, a transfer medium releasing position, and a transfer medium transporting section starting point position of the transfer medium transporting belt with which the leading end of the transfer medium initially comes into contact are disposed in an approximately virtual triangle. Therefore, the transfer medium passing through the transfer nip is moved on the moving pathway substantially along two sides of the approximately virtual triangle, that is, the side between the nip termination position of the transfer nip and the transfer medium releasing position and the side between the transfer medium releasing position and the transfer medium transporting section starting point position of the transfer medium transporting belt, by the rotation of the transfer roller and the image carrier.
However, when the leading end of the transfer medium reaches the transfer medium transporting section starting point position of the transfer medium transporting belt, the transfer medium is bent downward due to its own weight using the transfer medium transporting section starting point position and the nip termination position of the transfer nip as a supporting point, and is moved substantially along another side of the approximately virtual triangle between the transfer medium transporting section starting point position and the nip termination position. For this reason, the length of the moving pathway of the transfer medium is reduced, and thus the transfer medium is loosened and further bent downward. Then, the transfer image plane of the transfer medium passing through the transfer nip contacts the image carrier again, or contacts the members of the image forming apparatus arranged under the moving pathway of the transfer medium. As a result, the transferred image becomes disordered. In addition, the transfer position of the transfer medium varies subtly with the looseness of the transfer medium, and image deviation is generated by the variation of the width of the transfer nip. As seen from the above, just simply applying the technique for gripping the transfer medium disclosed in JP-T-2000-508280 to the image forming apparatus disclosed in JP-A-2009-205131 makes it difficult to obtain a good image.